(a) Field of the Invention
The present invention relates to a liquid crystal display and a driving method thereof.
(b) Description of the Related Art
Liquid crystal displays (LCDs) include two panels provided with pixel electrodes and a common electrode and coated with alignment layers and a liquid crystal (LC) layer with dielectric anisotropy, which is interposed between the two panels. The pixel electrodes are arranged in a matrix and connected to switching elements such as thin film transistors (TFTs). The switching elements selectively transmit data voltages from data lines in response to gate signals from gate lines. The common electrode covers entire surface of one of the two panels and is supplied with a common voltage. The pixel electrode, the common electrode, and the LC layer form a LC capacitor in circuital view, which is a basic element of a pixel along with the switching element connected thereto. Each pixel further includes a storage capacitor for enhancing the capacitance of the LC capacitor.
In the LCD, voltages are applied to the two electrodes to generate electric field in the LC layer, and the transmittance of light passing through the LC layer is adjusted by controlling the strength of the electric field, thereby obtaining desired images. In order to prevent image deterioration due to long-time application of the unidirectional electric field, polarity of data voltages with respect to the common voltage is reversed every frame, every row, or every dot.
When the LCD displays motion images or displays still images for a time interval, an afterimage is generated. The exemplary factors causing the afterimage are the concentration of ion impurity in the LC layer, the strength of the aligning force of the alignment layer, the kickback voltage, etc.
For example, ion impurity in the LC layer may be adsorbed due to inappropriate concentration thereof. The pixels are biased with a DC voltage generated by the ions even though there is no applied external field. The DC voltage affects the LC molecules to generate the afterimage.
The kickback voltage is a voltage drop before and after a voltage transition of a gate signal from a gate-on voltage for turning on the switching elements to a gate-off voltage for turning off the switching elements. The kickback voltage reduces both the positive data voltage and the negative data voltage to cause a DC voltage.
For reducing the afterimage, the concentration of the ion impurity in the LC layer is optimized, the aligning force of the alignment layer is maximized, and the kickback voltage is reduced.
A conventional technique for reducing afterimage generated due to the kickback voltage is to control the common voltage such that the voltages of the pixel electrode are symmetrical with respect to the common voltage. It is assumed that the positive data voltage and a negative data voltage for a gray to be applied to the pixel electrode are denoted as V+ and V−, respectively, and the kickback voltage is denoted as Vk. Then, the voltages of the pixel electrode are (V+−Vk) for the positive data voltage V+ and (V−−Vk) for the negative data voltage V−. The common voltage Vcom is determined by a following equation:(V+−Vk)−Vcom=Vcom−(V−−Vk).  (1)
However, it is difficult to make the common voltage Vcom satisfy Equation 1 for all grays and, if possible, the established common voltage may not remove the afterimage.